Method and system for handling network congestion

ABSTRACT

The present invention relates to a method and system for handling congestion in a packet data network, wherein at least two operating modes of congestion handling are selected based on a detected congestion level. The operating modes will then affect the applied congestion handling modes to thereby achieve a flexible and automatic network response which is based on the congestion level. Thereby applications&#39; quality requirements and service provider&#39;s business goals can both be met in an adequate manner.

FIELD OF THE INVENTION

The present invention relates to a method and system for handlingnetwork congestion in a packet data network, such as an IP network.

BACKGROUND OF THE INVENTION

Congestion in computer networks is becoming an important issue due tothe increased mismatch in links speeds caused by intermixing of old andnew technology. Recent technological advances have resulted in asignificant increase in the bandwidth of computer network links.However, these new technologies must coexist with the old low bandwidthmedia. This heterogeneity has resulted in a mismatch of arrival andservice rates in the intermediate nodes in the network causing increasedqueuing and congestion. If the load is small, throughput generally keepsup with the load. As the load increases, throughput increases. After theload reaches the network capacity, the throughput stops increasing. Ifthe load is increased any further, queues start building, potentiallyresulting in packets being dropped. Throughput may suddenly drop whenthe load increases beyond this point and the network is said to becongested.

In the following, congestion handling methods are methods that aim atreducing or avoiding network congestion as well as methods optimizingnetwork performance at the time of congestion. Thus, these methodscover, for example, a scheme which allows the network to operate at thepoint after which the increase in the throughput is small but results ina significant increase in the response time results. Another example isa scheme which tries to keep operating a network in the zone beforepackets start getting lost

Modern packet data networks handle network congestion in a relativelysimple-minded fashion. Typically, congestion is dealt with some packetdiscarding algorithm whose basic objective typically is to satisfy thequality requirements of applications. As to the packet discarding, thiscan be interpreted in a way that in the optimal case the packet lossratio of each individual flow is at most the packet loss parameterdefined by the characteristics of the application. From this viewpoint,it is usually assumed that during a congestion situation, the networknodes shall first discard packets with the loosest or lowest packet lossrequirement. However, if this principle is used extensively, networkresources are reserved or given primarily for flows and applicationswith the most stringent quality requirements. From the network andservice provider viewpoint, this result may be totally unacceptable. Forexample, a video stream with high quality of service (QoS) requirementsmay take the place of ten voice streams with lower QoS requirements,whereas the revenue obtained from the single video stream is typicallymuch lower than the revenue obtained from ten voice calls. Actually,what the service provider usually wants is either to divide the capacityevenly among the customers, or proportional to the fee paid by eachcustomer. These two targets, to serve applications quality requirements,i.e. individual benefit, and to meet the service provider's businessgoals, i.e. global benefit, are difficult to reach at the same time.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a methodand system for handling network congestion, by means of which a flexiblecongestion handling scheme serving both of the above targets in anadequate manner can be provided.

This object is achieved by a method of handling network congestion in apacket data network, said method comprising the steps of:

-   determining a congestion level in said packet data network;-   selecting based on said determined congestion level one of at least    two operating modes for performing said network congestion handling.

Furthermore, the above object is achieved by a system for handlingnetwork congestion in a packet data network, wherein said system isarranged to determine a congestion level in said packet data network,and to select based on said determined congestion level one of at leasttwo operating modes for said network congestion handling.

Additionally, the above object is achieved by a network element forhandling network congestion in a packet data network, said networkelement being arranged to determine a congestion level in said networkelement, and to select based on said determine congestion level one ofat least two operating modes for said network congestion handling.

Accordingly, network congestion handling can be optimized based on thedetected network congestion level. If the network congestion level issmall, it is possible to optimize the network congestion handlingalgorithm based on applications quality requirements in one operatingmode, because occasional congestion handling actions do not have asignificant effect on the sharing of network resources. In contrastthereto, if the network congestion level is high, other networkcongestion handling rules can be defined in another operating mode,wherein the total resources are divided between traffic flows. In otherwords, the operator supports the interests of individual applications aslong as the network congestion level is low enough and the businessobjectives of the operator are not threatened. Once the networkcongestion level gets worse, a new operating mode is selected to takeinto account the overall and not individual interests.

The at least two operating modes may be based on at least one of apacket discarding method, a method involving a reduction of trafficinserted to the network, and a method of shortening or compressing datapackets. The at least two operating modes may comprise a quality-basedoperating mode for controlling network congestion based on qualityrequirements of applications, and a quantity-based operating mode forcontrolling network congestion based on a desired sharing of resourcesbetween different flows and/or customers. In particular, thequality-based operating mode is preferably selected at a lowercongestion level, and the quantity-based operating mode is preferablyselected at a higher congestion level. If the quality-based operatingmode is set, the determined congestion level is checked, and theoperating mode is changed to the quantity-based operating mode if thecongestion level exceeds a first predetermined threshold level. On theother hand, if the quantity-based operating mode is set, the determinedcongestion level is checked and the operating mode is changed to thequality-based operating mode if the congestion level falls below asecond predetermined threshold level. Preferably, the secondpredetermined threshold level is lower than the first predeterminedthreshold level, to thereby provide a certain switching hysteresis bymeans of which excessive switching between operating modes can beprevented. The quality-based operating mode may be used as an initialdefault operating mode. Furthermore, the first and second predeterminedthresholds may correspond to first and second packet loss ratios,respectively.

The determination step may be performed by measuring the average packetloss ratio during a predetermined time period. In particular the averagepacket loss ratio may be an exponentially weighted moving average. As analternative, the determination step may be performed by applying aweighting scheme. In this case, packet loss values may be weighted basedon different classes defining maximum allowed packet loss ratios.

According to an advantageous further development, the at least twooperating modes may comprise a first operating mode applied below afirst congestion level and using no congestion handling methods, asecond operating mode applied between the first and a higher secondcongestion level and using a congestion handling method based on qualityrequirements of applications, a third operating mode applied betweensaid second and a higher third congestion level and using a congestionhandling method based on a reduction of a traffic amount inserted to thepacket data network, and a fourth operating mode applied above saidthird congestion level and using a congestion handling method based on adesired sharing of resources between different flows and/or customers.The congestion handling method of the second operating mode may beadapted to first discard data packets with highest allowed packet lossratios in a way that packet loss ratios of different quality categoriesremain on an acceptable level. Furthermore, the congestion handlingmethod of the third operating mode may be adapted to control adaptivetraffic sources in a way that a congestion situation is alleviatedbefore the congestion situation reaches said third congestion level.Moreover, the congestion handling method of the fourth operating modemay be adapted to divide the capacity of the packet data networkproportional to nominal bit rates.

The congestion level may be determined using at least one parameterrelated to said network congestion. The at least one parameter maycomprise packet loss ratios, filling levels of token buckets, bufferlengths in terms of number of packets or number of bytes, bufferingdelays for packets, measured bit rates of links and QoS thresholds.

The network congestion handling system may be arranged to determine thecongestion level in a predetermined network element, in a predeterminednumber of network elements, or in the whole packet data network. Thecongestion may be determined based on the whole traffic or only a subsetof traffic, e.g. some traffic class or classes in the network element,network elements or packet data network.

The network congestion handling network element may be a router or abuffer for an outgoing link in the packet data network.

Further advantageous developments are defined in the dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following the present invention will be described in greaterdetail based on preferred embodiments with reference to the accompanyingdrawings in which:

FIG. 1 shows a schematic architecture of a control system for handlingnetwork congestion, according to the preferred embodiments;

FIG. 2 shows a schematic diagram indicating different control domainsdepending on the network congestion level, according to the preferredembodiments;

FIG. 3 shows an arrangement of several operating modes for handlingnetwork congestion depending on the level of congestion;

FIG. 4 shows a congestion handling scheme based on four operating modesaccording to a first preferred embodiment;

FIG. 5 shows a flow diagram indicating a congestion handling processingbased on two operating modes according to a second preferred embodiment;

FIG. 6 shows a diagram indicating a time-dependent behavior of atwo-mode-system according to the second preferred embodiment; and

FIG. 7 shows a table indicating determined weighted congestion levels.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The preferred embodiments of the present invention will now be describedbased on a multimode congestion handling system for packet networks,such as IP networks.

FIG. 1 shows an architecture of a congestion handling scheme or anetwork with n users sharing it. The congestion state of the system isdetermined by the number of packets in the system. The total load at abottleneck resource corresponds to the sum of the individual loads ofthe users 1 to n. Usually, all resources demanded by the users 1 to nare granted. The network comprises a congestion handling function with acongestion control feature and a congestion avoidance feature.

The congestion avoidance feature generates a feedback information, e.g.a binary feedback, which controls traffic sources of the users 1 to n soas to increase or decrease their load. Thus the congestion avoidancefunction is arranged to prevent the network from becoming congested.

On the other hand, the congestion control function is arranged torecover the network from a congested state of high delay and lowthroughput. This may be achieved by discarding data packets or byshortening or compressing data packets.

According to the preferred embodiments, the congestion handling schemeis automatically selected by the network based on a detected congestionlevel to thereby achieve a flexible congestion response of the system.

FIG. 2 shows a schematic diagram of the basic principle underlying thepresent invention. In particular, different congestion handlingoperating modes are selected based on the network congestion level.These operating modes define a quality domain and a quantity domain oftheir congestion handling procedure. In the quality domain where thenetwork congestion level is small, the network congestion handlingalgorithm is optimized based on applications quality requirements,because occasional congestion handling actions such as packet discardingdo not have a significant effect on the sharing of network resources. Incontrast thereto, in the quantity domain where the network congestionlevel is high, network congestion handling rules essentially define howthe total resources are divided between the traffic flows of the users 1to n. Thus, in the quality domain, the operator supports the interestsof individual applications of the users 1 to n as long as the networkcongestion level is low enough and business objectives such asmaximization of revenue of the operator are not threatened. In thequantity domain where the network congestion gets worse, overallinterests of the users 1 to n are taken into account and trafficvolumes, i.e. quantity, are handled instead of individual quality.

In other words, at small network congestion levels, the optimalcongestion handling algorithm is based on applications qualityrequirements due to the fact that the effect of fulfilling the QoSrequirements of a single application of the users 1 to n does not haveany significant effect on the operator's business objectives. Incontrast thereto, at high network congestion levels, the target of thecongestion handling system is to realize a desired sharing of resourcesbetween different flows and/or users or customers according to theoperator's business objectives due to the fact that it is not reasonableanymore to try to optimize the QoS for single applications. Thereby, thepresent invention provides an advanced network congestion handlingsystem that takes into account both the applications' and the serviceprovider's business targets, as well as any other target that requiresspecific congestion handling mechanisms.

In particular, according to the preferred embodiments, the networkcongestion handling system may have two or more operating modes. Theselection of the operating mode may depend on the congestion situationin a single network element, a predetermined number of network elements,or the whole network. The congestion may be determined based on thewhole traffic or only some traffic class or classes in the networkelement, network elements or packet data network. The network congestionlevel may be expressed as one or a combination of parameters such as, abuffer occupancy level, measured bit rates of links, QoS thresholds(e.g. allowed priority of Simple Integrated Media Access (SIMA)),occupancy level of a token bucket and the like. The operating modes maybe based on congestion control functions such as packet discardingmethods or methods that shorten or compress IP packets etc., or may bebased on congestion avoidance functions such as methods that involve thereduction of traffic inserted to the network (e.g. methods having effecton the Transmit Control Protocol (TCP) flow control).

If the congestion handling system is arranged to perform the congestionhandling in a single network element, this may be an inner part of arouter or a buffer for an outgoing link, or any other unit that includesa mechanism to handle network congestion.

FIG. 3 shows a diagram indicating several operating modes for handlingnetwork congestion depending on the level of congestion. According toFIG. 3, the number of the operating mode increases with increasednetwork congestion level. Each of the operating modes 1 to n may bebased on specific congestion control and/or congestion avoidancefunctions thereby provide a flexible network response to any congestionsituation.

In the following, a four-mode system according to a first preferredembodiment and a two-mode system according to a second preferredembodiment are described with reference to FIGS. 4 to 6.

FIG. 4 shows a diagram indicating four specific operating modes M0 to M3according to the first preferred embodiment and their congestionhandling methods. In the operating mode M0, no congestion controlmethods, such as packet discarding or the like, are applied in case ofno or very low congestion below a first threshold level separating themodes M0 and M1. In the operating mode M1, a packet discarding algorithmis used which is solely based on the packet loss requirement of eachapplication without taking into account the bit rates of the flows.Above a second predetermined threshold level separating the operatingmodes M1 and M2, the intermediate operating mode M2 is selected whichmay include, for instance, a congestion avoidance algorithm which triesto reduce the amount of traffic inserted to the network by acorresponding feedback information allowing a TCP flow control.Additionally or alternatively, a mechanism for shortening or compressingdata packets may be applied in the intermediate operating mode M2. Abovea third threshold level separating the operating modes M2 and M3, acongestion handling mechanism is selected in the operating mode M3 so asto divide the capacity evenly among all customers or users by usinginformation about the momentary bit rates of the flows.

Thus, during lack of or low level of network congestion in the operatingmode M0, there is no need to apply any network congestion handlingactions, such as packet discarding. In this case applicationrequirements of the users 1 to n are met and the operator's overallbusiness objectives are not threatened.

During mild congestion in the operating mode M1, the system can be basedon a number of predefined categories each with a different maximumpacket loss ratio, for instance, 10⁻⁸, 10⁻⁶, 10⁻⁴, 10⁻². In theoperating mode M1, the network unit or network first discards packetswith the highest allowed packet loss ratio, in a way that at least inmost cases the packet loss ratio of each category will remain on anacceptable level. The information about the packet loss ratio can bemarked into the packet.

During intermediate network congestion in the operating mode M2,adaptive traffic sources may be controlled based on a feedbackinformation. For example, TCP flows may be handled in a way that thecongestion situation is alleviated before the congestion situationreaches a level in which the mode is changed to the operating mode M3.This kind of TCP flow control optimization may be based on an adjustmentof the TCP sliding window and other known TCP flow control methods. Asan alternative, the intermediate operating mode M2 may be arranged toutilize a congestion handling mechanism which involves the shortening ofthe length of the IP packets. In this case, the congestion is alleviatedby either discarding some parts of the IP packets or by applying acompression scheme to the IP header and/or the payload portion.

During a state of severe congestion in the operating mode M3, thecongestion handling system may effectively be used to divide the networkcapacity proportional to the nominal bit rates (NBR). This kind ofsystem is preferably when the congestion situation is so severe that thenetwork has to discard considerable amounts of packets.

FIG. 5 shows a flow diagram indicating a congestion handling processingin a two-mode system according to the second preferred embodiment. Thetwo operating modes consist of a first operating mode M1 for a mildcongestion situation and a second operating mode M3 for a severecongestion situation. Thus, in the second preferred embodiment, theintermediate operating mode M2 described in the first preferredembodiment has been omitted and the operating modes M0 and M1 of thefirst preferred embodiment have been merged together, while the samedenotation is used for the operating modes for mild and severecongestion, for reasons of clarity.

The packet discarding procedure shown in FIG. 5 is as follows. When apacket arrives at a network unit responsible for network congestionhandling, it is first checked whether the unit is in the first operatingmode M1. If so, a check is performed as to whether the congestion levelexceeds a threshold level L3. If this is the case, the operating mode ischanged to the second operating mode M3, in which the packet discardingmechanism is based on the quantity used by the flow. If the congestionlevel is below the threshold level L3, the first operating mode M1 ismaintained and a packet discarding mechanism based on qualityrequirements of the applications is used.

On the other hand, if the unit is not in the operating mode M1 when thepacket arrives, it is checked whether the congestion level is belowanother predetermined threshold level L1. If not, then a secondoperating mode M3 is maintained and a packet discarding mechanism basedon the quantity used by the flow is applied. On the other hand, if thecongestion level is below the other threshold level L1, the operatingmode is changed to the first operating mode M1 in which the packetdiscarding mechanism is based on the quality requirement of theapplication.

If there is a need to discard one or more packets, the algorithm of theproper discarding mode is applied, and the most appropriate packet to bediscarded is selected. Thus, the congestion handling system starts fromthe operating mode M1 as a default operating mode for a situationwithout any or low congestion.

FIG. 6 shows a diagram indicating a time-dependent behavior of thecongestion handling system according to second preferred embodiment.Before a time T1, the system starts in the first operating mode. M1 at avery low congestion. Then, the congestion or utilization level graduallyincreases and exceeds the upper threshold level L3 at the time T1. Atthis point in time, the operating mode is changed to the secondoperating mode M3. Then, after a while, the congestion level falls againbelow the other lower threshold level L1 and the operating mode ischanged back to the first operating mode M1. The difference between theupper threshold level L3 and the lower threshold level L1 defines ahysteresis suitable to prevent too frequent or excessive changes betweenthe operating modes.

The network congestion level measurement or detection may be performedby measuring the average packet loss ratio during a given period oftime. Any suitable measuring principle can be used, such as anexponentially weighted moving average which gives an average packet lossratio weighted with an exponential function. The congestion handlingsystem may be implemented in a way that the threshold levels L1 and L3in FIG. 6 correspond to two packet loss ratios, for instance 10⁻⁴ and10⁻². Then, the operating mode is changed to the second operating modeM3 for severe congestion when the average packet loss ratio exceeds 1%and returns to the first operating mode M1 for mild or no congestionwhen the packet loss ratio falls below 0.01%.

Another possible implementation is based on a weighted congestion level.In this case, the congestion level C can be based on weighted packetloss values as given by the following equation:

$C = \frac{\sum\limits_{i = 1}^{N}\;{B_{i} \cdot \frac{P_{{loss},i}}{P_{{req},i}}}}{\sum\limits_{i = 1}^{N}\; B_{i}}$where

-   P_(loss,i) denotes the measured packet loss ratio in a quality class    i (during the time intervall t),-   P_(req,i) denotes the maximum allowed packet loss ratio in the class    i,-   B_(i) denotes the incoming number of bytes in the class i, and-   N denotes the number of classes.

FIG. 7 shows a table indicating an example of calculations with threedifferent quality classes, wherein no congestion is determined for aclass 3 of highest quality requirements, and highest congestion isdetermined for an intermediate class 2. Due to the fact that the appliedweights increase with the number of received bytes in the respectiveclass, the impact of the high traffic class 1 packet loss ratio on thetotal congestion level is emphasized.

The principle presented in FIG. 6 may also be applied in this case ofdetermining a weighted congestion level. For instance, the upperthreshold level L3 in FIG. 6 may be “1” and the lower threshold level L1may be “0.1”. The difference between the threshold levels L3 and L1should be large enough due to the fact that the packet loss ratio mayvary very quickly. The dynamics of the system also depend on themeasuring interval which should be selected according to the desiredrequirements.

It is noted that the present invention may be applied in any packet datanetwork having a mechanism to handle network congestion. Furthermore,the at least two different operating modes may be based on anycongestion control or congestion avoidance mechanism suitable to achievethe desired flexible response of the network. Thus, the preferredembodiments may vary within the scope of the attached claims.

1. A method comprising: determining whether a packet data network isoperating in one of two operating modes; determining a congestion levelin said packet data network based on whether the packet data network isoperating in the one of the two operating modes; and selecting, based onsaid determined congestion level, one of the two operating modes forperforming a network congestion handling, wherein the two operatingmodes comprise one or more of a first operating mode, a second operatingmode, a third operating mode, a fourth operating mode, wherein the firstoperating mode is applied below a first congestion level and uses nocongestion control methods, the second operating mode is applied betweensaid first congestion level and a higher second congestion level anduses a second congestion control method based on quality requirements ofapplications, the third operating mode is applied between said secondcongestion level and a higher third congestion level and uses a thirdcongestion control method based on a reduction of a traffic amountinserted to said packet data network, and the fourth operating mode isapplied above said third congestion level and uses a fourth congestioncontrol method based on a sharing of resources between at least one ofdifferent flows and customers, wherein the sharing of resources dividesa capacity of said packet data network proportional to nominal bitrates, wherein the two operating modes comprise a quality-basedoperating mode configured to control a first network congestion based onquality requirements of the applications, and a quantity-based operatingmode configured to control a second network congestion based on thesharing of resources between at least one of the different flows andcustomers, and wherein said quality-based operating mode is selected ata lower congestion level, and said quantity-based operating mode isselected at a higher congestion level, wherein the selecting isimplemented by network element.
 2. A method according to claim 1,wherein said selecting the one of said two operating modes is based onat least one of a packet discarding method, a method involving areduction of traffic inserted to the network, a method of shortening orcompressing data packets, and a method of arranging more networkresources.
 3. A method according to claim 1, further comprising:changing to said quantity-based operating mode when said quality-basedoperating mode is set and said determined congestion level exceeds afirst predetermined threshold level.
 4. A method according to claim 3,further comprising: changing to said quality-based operating mode whensaid quantity-based operating mode is set and said determined congestionlevel is below a second predetermined threshold level.
 5. A methodaccording to claim 4, wherein said changing to said quality-basedoperating mode comprises changing to said quality-based operating modewhen said second predetermined threshold level is lower than said firstpredetermined threshold level.
 6. A method according to claim 5, whereinsaid first predetermined threshold level and said second predeterminedthreshold level correspond to a first packet ratio and a second packetloss ratio, respectively.
 7. A method according to claim 1, furthercomprising: using said quality-based operating mode as an initialdefault operating mode.
 8. A method according to claim 1, wherein saiddetermining said congestion level comprises using at least one parameterrelated to said network congestion.
 9. A method according to claim 8,wherein said using comprises using said at least one parametercomprising packet loss ratios, filling levels of token buckets, bufferlengths in terms of number of packets or number of bytes, bufferingdelays for packets, measured bit rates of links, and quality of servicethresholds.
 10. A method according to claim 8, wherein said usingcomprises using said at least one parameter comprising a measuredparameter of the whole traffic or a subset of traffic in said packetdata network.
 11. A method according to claim 10, wherein said usingcomprises using said at least one parameter comprising said subset oftraffic corresponding to predetermined traffic classes.
 12. A methodaccording to claim 1, wherein said determining the congestion level insaid packet data network comprises measuring an average packet lossratio during a predetermined time period.
 13. A method according toclaim 12, wherein said determining the congestion level in said packetdata network comprises measuring said average packet loss ratiocomprising an exponentially weighted moving average.
 14. A methodaccording to claim 1, wherein said determining the congestion level insaid packet data network comprises applying a weighting scheme.
 15. Amethod according to claim 1, wherein said determining the congestionlevel in said packet data network comprises applying said weightingscheme comprising packet loss values weighted based on different classesdefining maximum allowed packet loss ratios.
 16. A method according toclaim 1, wherein said selecting comprises selecting said one of the twooperating modes comprising said congestion control method of said secondoperating mode being configured to first discard data packets withhighest allowed packet loss ratios in a way that packet loss ratios ofdifferent quality categories remain on an acceptable level.
 17. A methodaccording to claim 1, wherein said selecting comprises selecting saidone of the two operating modes comprising said congestion control methodof said third operating mode being configured to control adaptivetraffic sources in a way that a congestion situation is alleviatedbefore said congestion situation reaches said third congestion level.18. An apparatus, comprising: a network element configured to: determinewhether a packet data network is operating in one of two operatingmodes; determine a congestion level in said packet data network based onwhether the packet data network is operating in the one of the twooperating modes; select, based on said determined congestion level, oneof the two operating modes to perform a first network congestionhandling, wherein the two operating modes comprise one or more of afirst operating mode, a second operating mode, a third operating mode, afourth operating mode, wherein the first operating mode is applied belowa first congestion level and uses no congestion control methods, thesecond operating mode is applied between said first congestion level anda higher second congestion level and uses a second congestion controlmethod based on quality requirements of applications, the thirdoperating mode is applied between said second congestion level and ahigher third congestion level and uses a third congestion control methodbased on a reduction of a traffic amount inserted to said packet datanetwork, and the fourth operating mode is applied above said thirdcongestion level and uses a fourth congestion control method based on asharing of resources between at least one of different flows andcustomers, wherein the sharing of resources divides a capacity of saidpacket data network proportional to nominal bit rates, wherein the twooperating modes comprise a quality-based operating mode configured tocontrol a network congestion based on quality requirements ofapplications, and a quantity-based operating mode configured to controla second network congestion based on a desired sharing of resourcesbetween at least one of different flows and customers, and wherein saidquality-based operating mode is selected at a lower congestion level,and said quantity-based operating mode is selected at a highercongestion level.
 19. An apparatus according to claim 18, wherein saidnetwork element is further configured to determine said congestion levelin a predetermined network element.
 20. An apparatus according to claim18, wherein said network element is further configured to determine saidcongestion level in a predetermined number of network elements.
 21. Anapparatus according to claim 18, wherein said network element is furtherconfigured to determine said congestion level in the whole packet datanetwork.
 22. An apparatus according to claim 18, wherein said congestionlevel is determined based on either the whole traffic or a subset oftraffic.
 23. An apparatus according to claim 18, wherein said apparatusis a router or a buffer for an outgoing link in said packet datanetwork.